1. Field of the Invention
The present invention relates to an ultrasonic stand-off gauge for use in measuring the instantaneous stand-off between the drill stem and the borehole wall while drilling subterranean oil and gas wells.
2. Description of the Related Art
Apparatus for measuring the inner diameter of a borehole is known in the art, where a borehole is a well bore drilled in the ground. One such apparatus is a mechanical caliper, which measures the diameter of the borehole by extending a plurality of mechanical arms or members until the members contact the inner surface of the borehole. The mechanical caliper traverses the borehole to provide the desired information regarding borehole diameter with respect to depth. An example of a mechanical caliper is disclosed in U.S. Pat. No. 4,876,672 to Petermann, et al.
Mechanical borehole calipers have several well known disadvantages and deficiencies. One such disadvantage relates to the steel casing utilized in a borehole to support the upper levels of the borehole. Actuation of the caliper arms against the side of the steel casing will often result in scratching and other damage to the casing. Still another disadvantage of such mechanical calipers are their relatively complex construction and the fact that they are not part of the drilling apparatus. Because the caliper is a separate probe, the drilling operations must cease when measurements are taken. In addition, mechanical calipers require a large number of measurements in order to sufficiently define or map the borehole diameter.
Acoustic type caliper devices are also well known and overcome many of the problems and disadvantages of mechanical calipers. An example of an acoustic type caliper device is disclosed in U.S. Pat. No. 4,827,457 to Seeman et al. The Seeman apparatus comprises a plurality of piezoelectric type transducers mounted on a sonde. A sonde is a wireline device lowered into the borehole after drilling operations have ceased. Each transducer transmits a pulse through borehole fluid in the borehole towards the borehole wall. The borehole fluid may also be referenced to as drilling fluid or mud, which is the medium through which the acoustic waves travel. The acoustic pulses are then reflected by the borehole wall and return through the drilling fluid back to the sonde, where they are detected by the transducer from which they were generated, or by another transducer dedicated to the receiving of such pulses. The elapsed time between the transmission and reception of each pulse, otherwise referred to as the round-trip transit time (RTT), can be employed to provide a measurement of the distance from the transducer to the inner surface or wall of the borehole. The RTT divided by the speed of the pulse through the drilling fluid provides the distance traveled by the pulse. This distance is twice the distance from the transducer to the borehole wall.
In the Seeman '457patent, the propagating velocity of the acoustic waves through the drilling fluid may be measured by use of a reference transducer. Nonetheless, the apparatus of Seeman employs nine transducers disposed in such a manner as to provide measurement over the entire inner circumference of the borehole. This is required because the sonde does not rotate. The processing of information from nine discrete transducers, as required by Seeman, necessitates complex multiplexer and demultiplexer circuitry. Further, the rate at which the sonde traverses the borehole determines the amount of information obtained. The slower the sonde traverses the well, the greater the amount of information obtained.
Other examples of wireline type acoustic calipers are described in U.S. Pat. Nos. 3,835,953 and 4,867,264 to Siegfried as well as U.S. Pat. No. 4,979,151 to Ekstrom, et al. All of these wireline acoustic caliper devices suffer from the distinct disadvantage of the necessity for pulling the drill stem from the borehole for utilization of the wireline tool. This operation is both time consuming and expensive from the standpoint of the drilling operation.
There have been attempts to take these measurements of the borehole during drilling operations. Devices to achieve this purpose are typically referred to as measurement-while-drilling (MWD) devices, which have the distinct advantage of allowing drilling operations to continue without interruption. Nevertheless, the sound speed of the drilling fluid must still be accurately determined.
The drilling fluid is preferably a special mixture of clay, water and chemical additives pumped downhole through a drill stem during drilling operations and upwardly through the annulus to return to surface drilling equipment. The drill stem typically includes a drill string connected to a series of drill collars, where the drill collars are connected to a drill bit at the end of the drill stem used to drill the borehole. The primary function of the drill collars is to provide a downward thrust or weight on the drill bit. The drilling fluid is pumped downwardly through a control bore extending through the string drill pipe and drill collars, and out of the drill bit. The drilling fluid cools the rapidly rotating bit, lubricates the drill stem as it turns in the well bore, carries rock cuttings to the surface and serves as a plaster to prevent the borehole wall of the borehole from crumbling or collapsing. The drilling fluid also provides the weight or hydrostatic head to prevent extraneous fluids from entering the borehole and to control downhole pressures that may be encountered.
An example of an MWD configuration is disclosed in U.S. Pat. No. 4,665,511 to Rodney, et al. This apparatus comprises at least one acoustic transceiver disposed within a section of a drill stem. The acoustic transceiver is adapted for generating an acoustic pulse and directing that pulse towards the borehole wall. The pulse is then reflected from the borehole wall and returns to the acoustic transceiver where the RTT is determined. A second acoustic receiver is disposed longitudinally with respect to the first transceiver at a selected distance for receiving a portion of the acoustic pulse generated by the transceiver. The difference in travel time between the pulse sensed by the second receiver and the pulse sensed by the transceiver is intended to be determinative of the acoustic velocity of the drilling fluid through which the pulses' have propagated, since the difference in distances of the travel paths of the respective pulses is considered known. The measured acoustic velocity of the drilling fluid is then combined with the RTT measured by the transceiver to compute stand-off. It should be noted that an erroneous measurement of the acoustic velocity of the drilling fluid will result in an erroneous stand-off determination.
The apparatus described in Rodney '511 can yield erroneous indications of the acoustic velocity of the drilling fluid because of the physical constraints present in the borehole and because of the directional nature of ultrasonic sensors. It is first noted that the directional path of the transmitted pulse as reflected off the borehole wall depends upon many factors, such as the strength of the signal and the condition of the borehole wall at the point of reflection. A longitudinally displaced receiver might not receive a reflected signal at all if the borehole wall is shaped so as to reflect the pulse in another direction, or if the tool is off-centered in the borehole and the pulse is not reflected in the direction of the receiver. More importantly, a longitudinally placed receiver is more likely to receive multiple reflections, where it is difficult to determine which path each reflection had traveled. Thus, the distance traveled by each reflected wave which is assumed in the teachings of Rodney to be known is, in fact, unknown in many conditions encountered in the actual MWD operation. An erroneous reflected wave path results in an erroneous acoustic velocity determination for the drilling fluid which, in turn, yields an erroneous stand-off measurement. Stated another way, the method of Rodney are subject to major error unless the borehole wall is very smooth, the MWD device is well centered within the borehole and acoustic impedances of the borehole environs are not conducive to multiple reflections. Such conditions in actual drilling practice is rather rare.
Furthermore, using devices according to Rodney, if the distance between the borehole wall and the transmitting device is relatively short compared to the distance between the transmitter and the longitudinally placed receiver, the wave will be "refracted" through the second medium of contrasting acoustic impedance. That is, refraction will occur at the interface of the drilling fluid and the borehole wall. In this manner, the wave essentially travels up the borehole wall before reaching the receiver, thus deviating from the simple ray path shown in Rodney. In any of these cases, erroneous or indeterminate results significantly reduce the effectiveness of the apparatus shown in Rodney. Since Rodney's device is only accurate in ideal conditions and does not provide accurate measurements of the acoustic velocity through the particular drilling fluid at a given time, it has been found that the Rodney device is ineffective for accurate stand-off measurements while drilling.
The acoustic stand-off measurement is complicated by several factors. One factor is the variation in the acoustic wave velocities (e.g. pressure wave velocity, shear wave velocity, etc. within the drilling fluid caused by differences in the density, pressure, temperature and composition of the drilling fluid. Other factors include the condition and shape of the borehole wall as well as the stand-off of the drill stem. These factors cause a relatively large dynamic range of received echo amplitudes. If the drilling fluid is water, which is rarely the case in most practical situations, the amplitudes of the reflected waves are relatively large and easy to measure. Heavy water-base muds cause greater attenuation of the propagating pulse, making echo detection more difficult. Oil-base muds of equal density to water-base muds cause a larger attenuation of the pulse, making echo detection even more difficult. The attenuation of these drilling fluids causes the echo to weaken as the path length, or stand-off, becomes larger. In addition, the amplitude of the echo can be smaller if the surface of the borehole wall is distorted or uneven.
There exists a need, therefore, for an accurate and reliable MWD stand-off measuring system for operation in a wide variety of conditions, which includes the capability of accurately measuring the velocity of acoustic waves within the drilling fluid at the same time the stand-off measurement is made. This acoustic velocity measurement is necessary in order to make an accurate stand-off measurement and must be independent of the condition of the borehole wall and impedance contrasts of the drilling fluid and the earth formation. An instantaneous stand-off measurement is desired because it may then be used to sort, in real-time, corresponding spectral data from formation evaluation tools, which is stand-off dependent. The data also provide further information on the effectiveness of the drilling program by indicating the movement of the drill stem in the borehole. To receive an accurate measurement of stand-off, an accurate determination of the acoustic velocity, or more specifically the velocity of pressure waves, within the drilling fluid is also required.